The present invention relates to an ignition plug for an internal combustion engine of an automobile. More specifically, the invention relates to an improvement in the structure of an ignition plug for an internal combustion engine of an automobile of a type to jet a flow of plasma gas caused by a generation of a spark between an inner electrode and an outer electrode into a combustion chamber so as to ignite an air-fuel mixture therein.
In order to enhance the efficiency of an internal combustion engine, in particular a gasoline engine, it is important to insure effective fuel ignition. Effective ignition is also an important means of satisfying the strict governmental regulations which have been imposed on the permissible levels of noxious substances contained in exhaust gases. However, in systems designed to burn thin air-fuel mixtures or in exhaust gas recirculation (EGR) systems, it becomes much more difficult to achieve effective ignition. Various attempts have been made at doing so. An example of one of such attempt is shown in FIG. 1 where in an ignition plug 1, an elongate electrode 2 is inserted at the center of a plug body. Reference numeral 3 denotes another cylindrical electrode which forms a part of the plug body in such a way as to encircle the elongate electrode 2. An insulator 4 such as ceramic or the like is provided in the plug body in such a way as to support the elongate electrode 2 and form a small cavity 5 at an end portion thereof. The cavity 5 is in communication with a combustion chamber by way of an opening 6 formed in the closed end wall of the cylindrical electrode 3.
In this ignition plug 1, a spark is generated between the elongate electrode 2 and the plug body 3 forming a cylindrical electrode, thereby creative a plasma gas of a high temperature and energy in the small cavity 5. The plasma gas thus created in the cavity 5 is jetted therefrom into the combustion chamber as a result of the pressure in the cavity 5 increasing due to thermal expansion of the plasma gas. The jetting plasma gas flow of a high temperature and energy brings about many flame cores in the air-fuel mixture in the combustion chamber, thereby ensuring the ignition without fail. In this ignition plug 1, however, there is a defect in that fuel, carbon, engine oil or the like can coat the inner wall of the cavity 5. When this occurs, a spark is not generated properly in the cavity 5, and a misfire results.
Another defect associated with the ignition plug 1 lies in the fact that the force of the plasma gas jet emerging from the cavity 5 is dependent solely on thermal gas expansion within the cavity.